Fluorescent powders are widely utilized in various fluorescent articles in our daily life, such as television cathode ray tube, display cathode ray tube, monitoring cathode ray tube, radar, flying-spot scanner, image-sensitivity enhancing device, cathode ray tube for copy machines, vacuum fluorescent display tube, electric plasma display, illuminating equipments, traffic signs, fluorescent plates, sensitivity-enhancing papers, and light-emitting diodes. Recently, due to the enhanced requirements of customers to image quality, such as resolution and lightness, and to illuminating effect, the researches on fluorescent powder are gradually noticed.
One importantly known fluorescent powder is yttrium aluminum garnet fluorescent powder having the chemical formula Y3Al5O12 (hereafter referred to as “YAG”). YAG is a compound consisting of Y2O3 and Al2O3 in the ratio of 3:5. Due to having specific laser optical properties, the compound is widely utilized in laser materials and fluorescent materials. YAG is a material having extremely high thermal stability. Because of wide bandgap energy, this material per se can emit ultra violet light (i.e. the short wavelength area). After being doped with rare earth metal elements, the emitting wavelength is then shifted to visible area (i.e. the long wavelength area). For illustration, YAG doped with cerium can emit yellow light, YAG doped with terbium can emit green light, and YAG doped with europium can emit red light.
YAG can be made by many conventional processes, such as solid state reaction process. The powders obtained by this process have a lager particle size, normally within the range between about 1 μm to tens of μm. Co-precipitating method and sol-gel method are commonly utilized in the art. The principle of co-precipitating method lies in adding a suitable precipitating agent, such as oxalic acid, citric acid, and carbonates, to allow the metal ions comprised in the aqueous solution containing metal ions such as cerium, yttrium, and aluminum ions to form complexes that are difficult to dissolve in water, at the same precipitating rate. Uniform fluorescent powder having small particle size can be obtained after filtration, heat-treatment and sintering at elevated temperature. A prior art technique pertaining to co-precipitation method can be seen in, for example, the article entitled “Ce3+, Fe3+-induced Optical Absorption in Ce,Fe:YAG Prepared by Co-precipitation,” (Jpn. J. Appl. Phys. Vol. 35. (1996) p.p. 1798-1801, Manabu et. al.). The principle of sol-gel method lies in mixing di-carboxylic acids and metal salts in polyol solutions to form metal alkoxides. After hydrolysis and thermal decomposition, fluorescent powder is obtained. A prior art technique pertaining to sol-gel method can be seen in, for example, the article entitled “Preparation and Characterization of Fine-Grain Yttrium-Based Phosphors by Sol-Gel Process,” J. Electrochem. Soc., Vol: 143, No. 1, January 1996, Ravi P. Rao).
However, fluorescent powders prepared by co-precipitation method normally comprise amorphous materials that contain large amount of hydroxy groups (—OH). During heat treatment, crystals will overly grow, which causes overly large particle size. When the resulting powders are applied to coating, unsatisfied uniformity and density result, which subsequently influences the fluorescent effect and resolution. Fluorescent powders prepared by sol-gel methods also have the disadvantage of the particle size being overly large.
Therefore, it becomes an important issue of the art to reduce the particle size of YAG fluorescent powders. Persons skilled in the art are working on producing the microparticles of YAG fluorescent powders. ROC (Taiwan) Patent Publication No. 412585 discloses a process for preparing fine particles of YAG fluorescent powders doped with cerium. The process is carried out by adding a chelating agent having both COOH ion and OH ion into an aqueous solution containing cerium, yttrium, and aluminum ions. After thermally treating the resulting viscous gel and removing the water contained in the gel, the gel is then subjected to sintering at a temperature between 1000 to 1500° C. However, the sintering temperature required in this known technique is still too high. This is disadvantageous to energy reduction, and causes increased production cost.
There is a need for a process that is simple and suitable for producing nano-scale YAG fluorescent powders in large scale.